This invention relates to a laser gauge interferometer which improves accuracy of measurement. This laser gauge interferometer can be used, for example, for improving locating accuracy of wafers in a semiconductor fabrication apparatus.
In a conventional laser gauge interferometer, the measurement error of displacement due to the change of the refractive index of air has been corrected in the following manner, as described in Japanese Patent Laid-Open No. 263801/1985. A reference beam as a beam for correcting the refractive index is disposed in the proximity of a signal beam separately from the signal beam and a reference beam reflecting mirror and a signal beam reflecting mirror are disposed in the spaced-apart relation by substantially the same distance. Since both beams are affected substantially equally by the change of the refractive index, the influences can be cancelled by taking the interference between booth beams and correction is thus made approximately. However, the prior art technique does not consider the measurement error occurring when the object of measurement moves in a certain large distance and the difference of the optical path lengths of both beams becomes great.
The prior art technique that has been most widely employed at present measures the temperature, pressure, humidity, etc., in the proximity of the signal beam by use of an environmental sensor and calculates the refractive index of air by numeric formulas to make numeric correction. However, since the number of measuring points in the environment is limited, correction can be made only on the average of the refractive index at the number of measuring points and is not complete as the correction covering the laser beam path as a whole.
As described above, in accordance with the prior art technique, correction becomes incomplete when the object of measurement undergoes displacement and the difference of the optical path lengths between the reference beam and the signal beam becomes great because the degree of influences of the change of the refractive index of air on both of these beams is different. Accordingly, the measurement error tends to become great.